Getting people to current content faster will keep them subbed longer.
Reason: People join the game when they do more often to play whatever the current content is.
When they find out they may need to spend a bunch of time just getting there, they will be more likely to quit.
Sure, getting a boost will get them there faster (which is why they include free ones with new xpacs), but speeding up leveling to get them to current faster also does that.
This is why they’ve done a level squish, along with taking the XP bonus from heirlooms (which not every player has) and baking it into the game for EVERYONE to access, and amping it up even more.